Many of the world’s top policy makers are rewriting their economic forecasts for the U.S., Europe, Japan and elsewhere, betting plummeting oil prices will lead to an overall boost in the global economy by delivering a windfall to consumers and manufacturers. Officials at the International Monetary Fund, U.S. Federal Reserve and European Central Bank have in recent days shrugged off concerns that the tumbling cost of crude signals a global slowdown. Instead, they project cheaper oil will be a shot in the arm for the world economy overall, especially countries with high energy tabs. Stanley Fischer , vice chairman of the U.S. Federal Reserve, called it a “supply shock” that will help the U.S. “It’s more likely to increase GDP than reduce it,” he said. “The effect is unambiguously positive,” European Central Bank President Mario Draghideclared after the bank’s monthly meeting last week. Some economists warn that the nearly 40% plunge in crude-oil prices in recent months is more a harbinger of gloom as Europe flirts with recession, Japan tries to recover from its own slump and China’s slowing growth risks morphing into a steeper pullback. Indeed, historically, sharp drops in oil prices tend to be associated with recessions as energy demand collapses.